問題描述:
本文將採用三次樣條插值及赫米特(Hermite)插值解決這一問題。
Sec 1
三次樣條插值的數學原理可自行百度
function Sanci(N)
%程序使用三次樣條插值
%N代表節點個數,數組形式
n=length(N);
for i=1:n
jiedian=linspace(-2*pi,2*pi,N(i));%插值節點的確認
%三次樣條插值
%計算函數值
for j=1:N(i)
F(j)=0.5*jiedian(j)-cos(jiedian(j));
end
%計算差商
deltx=diff(jiedian);
delty=diff(F);
firstorder=delty./deltx ;%一階差商
for k=1:length(jiedian)-2
delt2x(k)=jiedian(k+2)-jiedian(k);
end
delt2y=diff(firstorder);
D=delt2y./delt2x; %二階差商
%構造係數矩陣B
CN=length(jiedian);
for k=1:CN
for j=1:CN
if j==k
B(k,j)=2;
if k==1
B(k,k+1)=1;
end
if k==CN
B(k,k-1)=1;
end
if k>=2
B(k,j-1)=0.5;
B(k,j+1)=0.5;
end
if j~=k&&j~=k-1&&j~=k+1
B(k,j)=0;
end
end
end
end
%求解待定參數M
%根據手算得出邊界條件對應D值得表達式D=(6*(f_i-f_i-1)-3*h_i)/h_i^2
D_1=(6*firstorder(1)-12*pi/N(i))/(4*pi/N(i))^2;
D_Last=(6*firstorder(CN-1)-12*pi/N(i))/(4*pi/N(i))^2;
C=[D_1,D,D_Last];%附加邊界條件的新系數矩陣
M=(B'*C')';
for j=1:N(i)-1
x1=jiedian(j);x2=jiedian(j+1);
x_mid(j)=(x1+x2)/2;
%構造插值函數
S1(j)=(((x2-x_mid(j))^3)*M(j)+((x_mid(j)-x1)^3)*M(j+1))/(24*pi/N(i))+((x2-x_mid(j))*F(j)+(x_mid(j)-x1)*F(j+1))/(4*pi/N(i))-(4*pi*((x2-x_mid(j))*M(j)+(x_mid(j)-x1)*M(j+1)))/(6*N(i));
end
save('三次插值中點値.mat','S1');
save('z','x_mid');
subplot(3,2,i);
plot(x_mid,S1,'k-')%繪圖
hold on;
plot(x_mid,S1,'ko');%三次樣條插值點
hold on;
%繪製f=0.5*x-cosx圖形
sym x;
f='0.5*x-cos(x)';
h=ezplot(f,[-2*pi,2*pi]);
set(h,'color','r');
bt=strcat('節點=',num2str(N(i)));% 字符串連接
title(bt);
end
在命令行窗口輸入:
Sanci([7,14,21,56,112])
可得到下面的圖片,可以看到,插值節點越多,插值曲線與原曲線擬合程度越高
Sec 2
赫米特插值
function Hermite(N)
%程序使用赫米特插值
%N代表節點個數,數組形式
n=length(N);
for i=1:n
jiedian=linspace(-2*pi,2*pi,N(i));%插值節點的確認
%Hermite 插值繪圖
for j=1:N(i)-1
x1=jiedian(j);x2=jiedian(j+1);
x_mid(j)=(x1+x2)/2;
h1(j)=0.5*x1-cos(x1); %插值節點x1的函數值
h2(j)=0.5*x2-cos(x2); %插值節點x2的函數值
hx1(j)=0.5+sin(x1); %插值節點x1的導數值
hx2(j)=0.5+sin(x2); %插值節點x2的導數值
%插值函數值的計算
H(j)=(1+2*(x_mid(j)-x1)/(x2-x1))*(((x2-x_mid(j))/(x2-x1))^2)*h1(j)+(1+2*(x2-x_mid(j))/(x2-x1))*(((x_mid(j)-x1)/(x2-x1))^2)*h2(j)+((x_mid(j)-x1)*((x2-x_mid(j))^2)/((x2-x1)^2))*hx1(j)-((x2-x_mid(j))*((x_mid(j)-x1)^2)/((x2-x1)^2))*hx2(j);
end
save('Hermite插值中點値.mat','H');
subplot(3,2,i);
plot(x_mid,H,'b-');%繪圖
hold on;
plot(x_mid,H,'b*');%赫米特插值點
%繪製f=0.5*x-cosx圖形
sym x;
f='0.5*x-cos(x)';
h=ezplot(f,[-2*pi,2*pi]);
set(h,'color','r');
grid on;
bt=strcat('節點=',num2str(N(i)));% 字符串連接
title(bt);
end
在命令行窗口輸入:
>>Hermite([7,14,28,56,112])